In many printing systems the print head must be brought into contact with a print medium during the print cycle and removed from the print medium during the time the print head is being returned to the left margin.
This is particularly true where the printing system is of the non-impact type in which the print head or stylus electrically reacts with the print medium to cause actual printing, e.g., where the stylus comprises a plurality of electrodes and the print medium is coated with a conductive metal.
In this type of printing where the print stylus is removed from the print medium after a line of print is made and placed in contact with the print medium for printing a new line of print, it is critical that oscillations or vibrations of the print stylus be avoided because even very minute oscillations cause print deformations, e.g., character compression or expansion. Such deformations seriously affect the legibility of the printed characters. In addition the oscillations may damage the print stylus as well as the print medium which in turn may affect the print quality.
One chief source of these unwanted oscillations is a stylus actuator arrangement exhibiting high inertia which because of the degree of force required to move a print stylus into contact with a print medium causes the oscillations to occur when the stylus is stopped at the point where it contacts the print medium. The stop must be abrupt to assure that the print stylus stops just at contact with the print medium to avoid damage to the stylus or the print medium.
Typically, such a high-inertia system comprises a stylus carriage mounted to be actuated by a shaft by means of a motor-driven lever wherein the center of gravity of shaft and stylus carriage is at a distance from the area of activation, i.e., separated by the length of a lever. Thus, the center of mass is disposed at a relatively large radius from the axis of rotation which produces a large inertia. Thus, when the stylus actuating shaft is moved so that the print stylus contacts the print medium and is stopped, oscillations are induced in the shaft and, therefore, the stylus carriage and stylus as well, causing print deformations during part of the print cycle. The deformation problem is worse where the stylus is of the electrode type comprising a plurality of wire tips which must rub over the print medium during the print cycle.
The present invention contemplates a stylus actuator wherein the center of mass is concentrated near the axis of rotation, thereby substantially reducing the intertial forces opposing movement of the print stylus into contact with the print medium. More specifically, the present invention contemplates an actuating rod or shaft having a slightly offset axis of rotation. A print stylus-carrying structure is disposed for rotation by the shaft for moving the print stylus into printing contact with the print medium. The eccentricity of the actuating shaft provides sufficient displacement to actuate the stylus-carrying structure while making it possible to actuate the stylus with a relatively small force due to the low inertia offered by the actuating shaft. Thus, oscillation of the actuating rod and, therefore, of the print stylus when it is abruptly stopped at the print position is eliminated. This eliminates print-character distortion due to inertia-induced oscillations of the actuating shaft.